The present invention relates to a piezoelectric motor. One field of application of the invention is aircraft actuators.
Such electric motors are intended to replace conventional electromagnetic motors, since the latter generate electromagnetic interference which interferes with the avionics and the electrical control devices of the aircraft. Such piezoelectric motors of the PZT type are known with small powers, about a few watts. These motors generate a high torque and a low speed of around 100 revolutions per minute. However, they are not suitable for higher powers of around several kilowatts, the heating at these powers being very high and destructive. In fact piezoelectric materials, beyond a certain temperature, lose their piezoelectric properties, namely their ability to deform under the action of an electric field, in a similar manner to that of magnets, which lose their magnetic properties beyond the Curie point.
The inventor has found that this phenomenon of depolarisation occurs when, the piezoelectric component being on one of the branches of its hysteresis cycle, the value of the electric field to which it is subjected approaches a threshold situated on this branch at a certain distance from the value for which the polarisation is cancelled out. And the higher the temperature, the more this distance increases, and consequently the lower is the excursion domain available for subjecting the component to an alternating control voltage. The invention aims to obtain a piezoelectric motor remedying these drawbacks and functioning in a satisfactory manner over a wide power range, and therefore temperature range.
The object of the invention is a piezoelectric motor having:
a rotor having at least one disc rotating about an axis;
supply means for supplying an electric control voltage;
a stator having a set of at least one member for driving the disc in rotation, the drive member being able to be applied in a normal direction against the surface of the disc by a clutch member, and being able to be moved in a tangential drive direction by the deformation of at least one piezoelectric element able to be deformed whilst being subjected to a variation in the said electrical control voltage, characterised in that the said supply means are arranged to supply an electrical control voltage having a DC component of shifting towards the expansion state of the piezoelectric elements in the tangential direction.
Thus the invention makes provision for maintaining the piezoelectric elements in an operating zone where they keep their polarisation and a non-zero deformation. The variation in control voltage and the DC voltage are such that the polarisation of the piezoelectric elements remains greater than a non-zero threshold. According to one characteristic, the supply means are arranged to supply a DC component of the control voltage such that the control voltage keeps the same sign.
In order to obtain a greater margin of safety with respect to a depolarisation, the supply means are arranged to supply a DC component of the control voltage such that the control voltage is not cancelled out and its absolute value is greater than a strictly positive threshold. In order to isolate the DC voltage source producing the DC component of the control voltage with respect to the source producing the variable component of the voltage, the supply means are arranged to supply the DC component of the control voltage at a first terminal supplying voltage to the piezoelectric element by means of a decoupling inductor, and the first voltage supply terminal is connected, by means of a decoupling capacitor, to supplying voltage by the variable component of the control voltage. In order to compensate for the reactive energy generated by the piezoelectric element, an inductor is interposed in parallel with the piezoelectric element, on an input feeding by the variable component of the control voltage. In order to obtain a gain in voltage which is stable in a range of frequencies on the input feeding by the variable voltage component with respect to a variable voltage supply, the supply means are arranged to supply the variable component of the control voltage by means of a series LC circuit to this feeding input. According to one characteristic, the supply means are arranged to supply a variable component of the sinusoidal control voltage.
In an embodiment making it possible to increase the power of the motor, two piezoelectric elements are provided in front of and behind the drive member for its tangential movement. In a first particular embodiment, making it possible to optimise the power, the supply means are arranged to supply, to the piezoelectric elements of the drive member, variations in voltage which are substantially in phase opposition, for example with the same absolute values, and continuous voltage components substantially with the same sign, for example equal. The piezoelectric elements are then polarised in the same direction. In another particular embodiment, making it possible to use only one variable voltage source, the supply means are arranged to supply to the piezoelectric elements of the drive member variations in voltage which are substantially in phase, and for example equal, and DC voltage components of opposite sign, and for example with the same absolute values. The piezoelectric elements are then polarised in the reverse direction.
In a particular embodiment, improving the functioning of the motor, the stator has a complementary drive member on the other side of the surface of the disc, opposite the first drive member, and a member for prestressing the complementary drive member in the normal direction against the surface of the disc. In order to increase the power further, several drive members are distributed over the surface of the disc, the piezoelectric elements of adjacent drive members being connected together by a strut.
A particularly compact and powerful embodiment provides for the rotor to have a second disc coaxial with the first disc and for the stator to have a second set of drive members, at least one clutch member being provided between the two discs in order to apply at least the first and second drive members in opposite directions in the normal direction against the surfaces of the first and second discs. First and second complementary drive members are provided on the other side of the surface of the discs, opposite the first and second drive members and the prestressing members for the first and second complementary drive members in the normal direction against the surface of the discs. An inexpensive embodiment provides for the prestressing members to be formed by springs. The invention will be more clearly understood in the light of the following description made with reference to the drawings, given solely by way of example and in which: